Transmission of a 2009 pandemic influenza virus shows a sensitivity to temperature and humidity similar to that of an H3N2 seasonal strain

Abstract

In temperate regions of the world, influenza epidemics follow a highly regular seasonal pattern, in which activity peaks in midwinter. Consistently with this epidemiology, we have shown previously that the aerosol transmission of a seasonal H3N2 influenza virus is most efficient under cold, dry conditions. With the 2009 H1N1 pandemic, an exception to the standard seasonality of influenza developed: during 2009 in the Northern Hemisphere, an unusually high level of influenza virus activity over the spring and summer months was followed by a widespread epidemic which peaked in late October, approximately 2.5 months earlier than usual. Herein we show that aerosol transmission of a 2009 pandemic strain shows a dependence on relative humidity and temperature very similar to that of a seasonal H3N2 influenza virus. Our data indicate that the observed differences in the timings of outbreaks with regard to the seasons are most likely not due to intrinsic differences in transmission between the pandemic H1N1 and seasonal H3N2 influenza viruses.

FIG. 1.

NL/09 virus transmitted as an aerosol exhibits a sensitivity to RH and temperature similar to that of Pan/99 virus. Viral titers in nasal washes are plotted as a function of day postinoculation. Titers from inoculated animals are represented by dashed lines with squares, and titers from exposed animals are shown with solid lines and triangles. The environmental conditions under which each experiment was conducted are stated at the left, and the result in terms of number of exposed animals which contracted infection over the 7-day exposure period is indicated underneath each graph. Results obtained with the pandemic strain NL/09 virus are shown on the left, while results obtained with the seasonal strain Pan/99 virus are shown on the right. Experiments shown in panel B were performed at different times; experiments shown in panels A, C, and D were done in parallel.